Spellbreak

Mouse Sensitivity Calculator and Converter

For a great collection of info, see Skidushe's excellent thread.
Also check out these instructional videos to get started; One covering the basics, and one covering Monitor Distance in details:

Version 7.5.g (January 20, 2019) - This is a brand new version with support for multi-calculations, see this thread for info!
The old version is available here so you can check if you encounter a bug.
New games: FPoSu, Atlas, Just Cause 4, Supraland, Hitman 2, Spellbreak, Creative Destruction, KovaaK's FPS Aim Trainer, The Long Dark, Fallout 76, Project Warlock.